Sodium lauryl sulfate (SLS) is a cleaning and foaming agent found in everything from shampoo to toothpaste to dish soap. It works by breaking down oils and grease so they can be rinsed away with water. If you’ve ever wondered why it appears on so many ingredient labels, the short answer is that it’s cheap, effective, and creates the rich lather people associate with a thorough clean.
How SLS Actually Works
SLS is a surfactant, meaning it reduces the surface tension of water. Water on its own beads up and rolls off oily surfaces. SLS molecules have a split personality: one end attracts water, the other attracts oil. When you add SLS to water, these molecules migrate to the surface and cluster together, allowing the liquid to spread more evenly and mix with substances it normally can’t touch.
When enough SLS molecules are present, they form tiny spheres called micelles. The oil-loving ends face inward, trapping grease and dirt in the center, while the water-loving ends face outward. This is the basic mechanism behind all detergents, and it’s why SLS can pull sebum off your scalp, food grease off a plate, or plaque-forming bacteria off your teeth. The foaming you see is a byproduct of this process, not actually what does the cleaning.
Where You’ll Find It
SLS shows up in an enormous range of consumer products. In household cleaning, it’s used at concentrations of 0.5% to 2% in dish soaps, surface cleaners, and laundry detergents. In personal care, it’s a primary ingredient in shampoos, body washes, hand soaps, and facial cleansers.
In dentistry, SLS has been added to toothpastes for over 50 years. It helps dissolve the paste’s other active ingredients and spreads them evenly across your teeth during brushing. The foaming action also helps the toothpaste reach between teeth and along the gumline. Beyond cleaning products, SLS is used in some pharmaceutical formulations as an emulsifier, helping blend ingredients that wouldn’t otherwise mix.
Skin and Scalp Irritation
SLS is not a gentle ingredient. It’s actually used as a standard irritant in dermatology research precisely because it reliably causes a skin reaction at known concentrations. A 1% SLS solution applied to skin for 24 hours produces concentrations in the outer skin layer that exceed the threshold for triggering irritation responses like redness, dryness, and itching.
For most people using a shampoo or body wash, this isn’t a practical concern. The product is on your skin briefly and then rinsed off, keeping actual exposure low. But if you have eczema, rosacea, or generally sensitive skin, SLS can strip away enough of your skin’s natural oils to leave it dry, tight, or inflamed. The same applies to your scalp: SLS is very efficient at removing sebum, which is great for cutting grease but can leave already-dry hair feeling brittle and stripped.
Studies using radiolabeled SLS on animal skin found that the compound deposits heavily on the skin surface and inside hair follicles. At high concentrations, this deposition could theoretically damage the follicle. However, no evidence has shown that SLS at the levels found in consumer products causes hair loss. The Cosmetic Ingredient Review panel recommends that leave-on products (lotions, creams, styling products) contain no more than 1% SLS, while rinse-off products like shampoo and body wash are considered safe for brief use followed by thorough rinsing.
Effects on Canker Sores
One of the more practical reasons to care about SLS is its connection to canker sores. Research published in the British Dental Journal found that people prone to recurrent canker sores (aphthous ulcers) experienced fewer episodes, shorter duration, and less pain when they switched to SLS-free toothpaste. If you get canker sores regularly, this is one of the simplest changes you can make. Several major toothpaste brands now offer SLS-free formulas specifically for this reason.
SLS vs. SLES
Sodium laureth sulfate (SLES) is a close chemical relative of SLS. It’s made by taking SLS and running it through a process called ethoxylation, which adds extra molecular units to the chain. This makes SLES a larger molecule that doesn’t penetrate skin as easily, resulting in a noticeably milder ingredient. Many shampoos and body washes have switched from SLS to SLES for this reason.
The trade-off is that the ethoxylation process can introduce trace amounts of 1,4-dioxane, a known carcinogen. Manufacturers typically use purification steps to remove it, and finished products generally contain only very low levels. Still, if you’re choosing between the two, neither is inherently “safe” or “dangerous.” SLS is harsher on skin but chemically simpler. SLES is gentler but carries a contamination concern during manufacturing.
Is SLS a Carcinogen?
No. Despite persistent internet claims, SLS has not been classified as a carcinogen by any major health authority. It is not listed by the International Agency for Research on Cancer or the National Toxicology Program. A comprehensive review of SLS toxicity data concluded there is evidence for safe use in household cleaning products at typical concentrations. The confusion likely stems from SLS being lumped together with SLES in online discussions, where the 1,4-dioxane contamination risk (which applies to SLES, not SLS) gets misattributed.
SLS is an irritant, not a toxin. The distinction matters. It can cause redness and dryness with prolonged or concentrated exposure, but it does not accumulate in your body or cause systemic harm at the levels present in consumer products. If you have sensitive skin, a history of canker sores, or color-treated hair that dries out easily, switching to SLS-free products is a reasonable choice. For everyone else, the ingredient does exactly what it’s designed to do: clean effectively and rinse away.